# Deferred Request Callbacks

One of the design principles of Flask is that response objects are created and passed down a chain of potential callbacks that can modify them or replace them. When the request handling starts, there is no response object yet. It is created as necessary either by a view function or by some other component in the system.

But what happens if you want to modify the response at a point where the response does not exist yet? A common example for that would be a before-request function that wants to set a cookie on the response object.

One way is to avoid the situation. Very often that is possible. For instance you can try to move that logic into an after-request callback instead. Sometimes however moving that code there is just not a very pleasant experience or makes code look very awkward.

As an alternative possibility you can attach a bunch of callback functions to the `g` object and call them at the end of the request. This way you can defer code execution from anywhere in the application.

## The Decorator

The following decorator is the key. It registers a function on a list on the `g` object:

    from flask import g

    def after_this_request(f):
        if not hasattr(g, 'after_request_callbacks'):
            g.after_request_callbacks = []
        g.after_request_callbacks.append(f)
        return f

## Calling the Deferred

Now you can use the after_this_request decorator to mark a function to be called at the end of the request. But we still need to call them. For this the following function needs to be registered as `after_request()` callback:

    @app.after_request
    def call_after_request_callbacks(response):
        for callback in getattr(g, 'after_request_callbacks', ()):
            callback(response)
        return response

## A Practical Example

At any time during a request, we can register a function to be called at the end of the request. For example you can remember the current language of the user in a cookie in the before-request function:

    from flask import request

    @app.before_request
    def detect_user_language():
        language = request.cookies.get('user_lang')
        if language is None:
            language = guess_language_from_request()
            @after_this_request
            def remember_language(response):
                response.set_cookie('user_lang', language)
        g.language = language